The present invention relates generally to machine vision systems and, ore specifically, to an enhanced vision system (EVS) for use in the piloting of aircraft. The invented system uses detectors sensitive to infrared radiation to generate a navigational display, preferably graphically representing the surrounding background scene such as terrain and structures, selected navigational references sensed by the EVS, and related information from other components of the overall navigation system for an aircraft. The preferred graphical representation is a fusion of enhanced, realistic camera images with computer-generated, scene-depicting symbology that is particularly helpful during approach and landing of an aircraft.
Vision systems are particularly valuable for the piloting of aircraft because aircraft are expected to fly in very diverse weather conditions, and because any error in navigating an aircraft can have extremely dire consequences. Poor visibility is often associated with flying in fog, but other atmospheric conditions severely limit visibility, including snow, rain, smoke, and ash. Discussion of the optical characteristics of the atmosphere and their impact on what is known as runway visual range is found in David C. Burnham et al., "United States Experience Using Forward Scattermeters for Runway Visual Range," U.S. Department of Transportation Report No. DOT/FAA/AND-97/1 DOT-VNTSC-FAA-97-1 (March, 1997), the disclosures of which are incorporated herein by reference. Furthermore, while the present invention is described with specific reference to EVS in aircraft, it is envisioned that the systems of the invention may be applied to other applications, including the use of machine vision in automobiles, as described in U.S. Pat. No. 5,161,107, the disclosures of which are incorporated herein by reference.
Various vision systems are disclosed in U.S. Pat. Nos. 4,862,164, 5,534,694, 5,654,890, 5,719,567, and in (1) Le Guilloux and Fondeur, "Using image sensors for navigation and guidance of aerial vehicles," International Society for Optical Engineering (SPIE) Proceedings, Vol. 2220, pp. 157-168; (2) Roberts and Symosek, "Image processing for flight crew enhanced situation awareness," International Society for Optical Engineering (SPIE) Proceedings, Vol. 2220 pp. 246-255; (3) Johnson and Rogers, "Photo-realistic scene presentation: `virtual video camera`," International Society for Optical Engineering (SPIE) Proceedings, Vol. 2220, pp. 294-302; (4) Dickmanns et al., "Experimental Results in Autonomous Landing Approaches by Dynamic Machine Vision," International Society for Optical Engineering (SPIE) Proceedings, Vol. 2220, pp. 304-313; and (5) Mostafavi, "Landing trajectory measurement using onboard video sensor and runway landmarks," International Society for Optical Engineering (SPIE) Proceedings, Vol. 2463, pp. 116-127, the disclosures of which are incorporated herein by reference. Specific detectors for use in EVS are found in U.S. Pat. Nos. 5,808,350, 5,811,807, 5,811,815, and 5,818,052, the disclosures of which also are incorporated herein by reference.
The generated imagery of the present EVS may be displayed on a head-up display, but head-down or other displays are within the scope of this invention. Head-up displays typically are used for pilot control of an aircraft during landing, and head-down displays typically are used for pilot monitoring of automatic landing system performance.
The vision system of the present invention preferably generates a display based on a fusion of images from two imagers. One of the imagers senses short-wavelength infrared radiation (SWIR), and the other senses long- or medium-wavelength infrared radiation (LWIR or MWIR). Each imager includes a detector and electronics to process a signal produced by the detector. The imagers may share optics, or may each have separate optics. The imagers are described as separate items because this is believed the best way to implement the invention using current detector and optics components. However, the imagers or selected components of the imagers may be integrated into a single optics/detector/electronics devices in the future. For example, several of the incorporated patents disclose integrated detector devices, sensitive to two separate ranges of radiation.
By processing two ranges of IR wavelengths separately, a broad dynamic range may be allocated to the signal generated by each of the detectors, without concern for the dynamic range required by the other of the detectors. Signal conditioning and processing by each imager may be optimized for sensing and imaging details of particular radiation sources within a range of IR wavelengths. The conditioned and processed signals from the two imagers then are adjusted relative to each other so that the image of he radiation sources within both sensed ranges of IR wavelength may be fused without losing image detail of either of the imaged ranges of IR wavelengths.
An SWIR imager generates an image of electric light sources. The preferred detector has limited sensitivity to IR radiation wavelengths above approximately 1.7-microns. Electric navigation lights emit strongly within the 1.5-micron to 1.7-micron ange of wavelengths, and there is relatively little unwanted background solar radiation within this range. Accuracy is improved by spectrally filtering any radiation sensed by the SWIR detector using a filter having a cut-on wavelength of approximately 1.5-microns. Because of this, a sharp, well-defined image of navigation lights may be generated, even in bright daylight fog or other obscurant.
The sensitivity of the SWIR imager may be increased during non-daylight use by lowering the cut-on wavelength of the filter to approximately 1-micron to allow a broader spectrum of radiation to its detector. For current uncooled radiation detectors, sensitivity below 1-micron wavelengths is limited, so there is no need for a spectral filter at night. Furthermore, as uncooled radiation detectors are improved, it may be desirable to decrease the non-daylight cut-on wavelength to approximately 0.4-microns.
Similarly, future uncooled detectors sensitive to SWIR radiation may be sensitive to wavelengths longer than 1.7-microns. It is believed that sensitivity of a detector to radiation wavelengths up to approximately 2.35-microns would enhance system performance. Sensitivity to longer wavelengths than 2.4-microns may require a filter having a daylight and non-daylight cut-off wavelength of approximately 2.4-microns to limit the amount of background radiation sensed by the detector.
The preferred SWIR imager further includes a signal processor that identifies the center of each perceived radiation source within its specific wavelength range. The relative location of each perceived radiation point source then is mapped to a display, so that a precise dot or series of dots is displayed. It is believed that such a mapped pinpoint display is more useful to a pilot in navigation than a simple direct display of the perceived radiation sources, because the radiation sources tend to be sensed as relatively diffused blots or blurs that are difficult to interpret visually. Furthermore, the diffused blots or blurs may be large enough to block or washout other imagery that needs to be displayed, as discussed below.
A preferred second imager senses long wavelength infrared radiation in the range of 8- to 14-microns in wavelength, to generate an image of the surrounding background scene such as runway edges, runway markings, terrain, structures and vehicles. Long-wavelength infrared radiation in this range of wavelengths has been found to have excellent transmissivity through fog and some other atmospheric conditions, and represents the peak spectral thermal emission of the background scene in cool ambient conditions. However, it does not include much radiation emitted by most navigation lights, so navigation lights do not show up well in images generated by the LWIR imager. A benefit is that navigation lights do not cause blooming or other interference in the images generated, or require any substantial portion of the dynamic range. Alternatively, the second imager can sense medium wavelength infrared radiation, in the range of 3- to 5-microns in wavelength, but this radiation tends to have less transmissivity through fog and other obscurants.
As described below, the image generated by the SWIR imager is relatively simple, with only a pattern of dots displayed. It is believed that this image may be displayed on a head-up display without undue distraction of a pilot, even in good visibility conditions. The background scene image generated by the LWIRJMWIR imager, on the other hand, may be distracting when good visibility conditions allow a pilot to see the relevant background scene without enhanced vision. Accordingly, the EVS of the present invention also may include a CCD visible light imager that monitors visibility conditions, and modifies image generation to minimize pilot distraction.
Further improvements to the images generated by the present EVS include enhancing the image based on predefined databases of patterns and features expected to be imaged. Object recognition may be used to identify recognizable patterns or features, and a computer-generated image may be fitted to the sensed image to add missing details. For example, varying atmospheric conditions may allow the EVS to sense only a portion of the radiation sources, or only sense them intermittently. Object recognition and computer-generated imagery is then used to fill in the missing details. Object recognition may also be used to improve navigational accuracy, by calculating a real-world position based on the location of identified patterns and features.
The advantages of the present invention will be understood more readily after a consideration of the drawings and the Detailed Description of the Preferred Embodiment.